When you sandwiched your quilt, did you have your backing fabric laying nice and taught before pinning? Have you checked your bobbon tension? Take the bobbin out, grab the thread, and let the bobbin drop. You should be able to shake the thread, and have the bobbin just fall slowly. If it is not falling, adjust the tension screw on the bobbin case til it falls just right. Try re-threading your machine, and changing needles.